A telescopic optical system including, in order from an object to be observed, an objective optical system, a relay optical system, and an eyepiece optical system is known. In this telescopic optical system, the relay optical system is provided with a plurality of movable lens groups, for example, and configured so as to vary magnification of the whole telescopic optical system by relatively moving each lens group and changing a focal length (composite focal length) (for example, see Patent Document 1).
In the telescopic optical system provided with such a zoom function (hereinafter, referred to as a variable-power optical system), a first intermediate image to be imaged in the objective optical system is reimaged as a second intermediate image by the relay optical system mentioned above, and further magnified by the eyepiece optical system. Here, positions of two image surfaces on which the first intermediate image and the second intermediate image are formed do not change, and therefore if a reticle is arranged on at least one of the two image surfaces, the variable-power optical system can be utilized as a collimation optical system.